Polyimides generally have excellent heat resistance, mechanical properties and electric properties. For this reason, polyimides are widely used as e.g., a molding material or a composite material, for various applications including electric/electronic materials and optical materials.
Of these polyimides, polyimides obtained by reacting an alicyclic diamine with an aromatic tetracarboxylic acid have relatively high transparency and future applications thereof to uses including electric/electronic materials and optical materials have been looked forward to (for example, see Patent Literature 1 and Patent Literature 2).
These polyimides with excellent transparency are being studied for uses, for example, in circuit substrates such as suspension substrates for HDD, semiconductor package substrates and substrates for flexible display. These circuit substrates generally have a polyimide resin layer that has been patterned. Since mounting of various electronic parts on these circuit substrates needs heating operation, the polyimide resin layer is required to have heat resistance. With recent environmental concern in particular, a solder used in electronic circuits has been mainly lead-free, and accordingly the solder reflow temperature shifts toward the higher temperature side. It has been therefore demanded to obtain polyimides with higher heat resistance than conventional ones, for example, with a glass transition temperature (Tg) of not less than 260° C.
In general, the patterning of the polyimide resin layer is performed by exposing via a photomask a photosensitive resin layer containing an ultraviolet ray polymerizable compound which is formed on the polyimide resin layer, the transparency of the polyimide being taken advantage of, and then performing development (etching) treatment using an alkali solution. As such, desired are polyimides having not just light transmittance but also high ultraviolet ray transmittance.
In use of the polyimides as circuit substrates with their space for electric/electronic parts being narrow or complicatedly-shaped for the mounting of various electronic parts, the polyimides are expected to be used in parts requiring flexibility (e.g., parts connecting a head portion with a body of a printer), and therefore are required to be excellent in flexibility, too.